The present invention relates to a wall made from a composite material reinforced so as to limit the spread of a crack in a given direction.
To limit the impact of noise annoyance in aircrafts, techniques have been developed to reduce the noise, in particular by positioning panels or coatings at certain walls aiming to absorb some of the sound energy, in particular using the principle of Helmholtz resonators.
This type of panel comprises, from the outside toward the inside, an acoustically resistive porous layer, at least one cellular structure, and a reflective or impermeable wall. Alternatively, the panel may comprise several superimposed cellular structures between which acoustically resistive porous layers are provided. The cells of the cellular structure(s) are sized so as to ensure optimal acoustic treatment.
“Layer” refers to one or more layers that may or may not be of the same nature.
According to one embodiment, the cellular structure assumes the form of one or more honeycombs made from a composite material.
The reflective wall may be made from a composite material and obtained by draping fibers embedded in a resin matrix.
The acoustically resistive structure is a porous structure playing a dissipative role, partially converting the acoustic energy of the sound wave passing through it into heat. It comprises open areas capable of allowing the acoustic waves to pass, and other closed or solid areas not allowing the sound waves to pass, but designed to ensure the mechanical strength of said layer. This acoustically resistive layer is in particular characterized by an open surface rate that varies essentially as a function of the engine, the components making up said layer.
In general, the acoustically resistive structure comprises at least one porous layer and at least one reinforcing structure.
The porous layer must make it possible to make the acoustic treatment linear and trap the acoustic waves in the Helmholtz cells formed by the cellular structure.
According to one embodiment, the porous layer is a metal fabric, in particular a wire mesh with metal wires having a diameter in the vicinity of 0.1 mm.
According to one advantage, this metal fabric is an excellent conductor for sweeping lightning.
According to one constraint, this wire mesh, which is in contact with the flows of air, must not generate excessive protrusions, such that the diameter of the wires is limited and less than 0.1 mm or 0.2 mm. Beyond that, the wires of the mesh would create aerodynamically prohibitive protrusions.
According to one important point, part of the surface of the metal wire mesh is glued to the other layers of the acoustically resistive structure.
The reinforcing structure assumes the form of a plate made from a composite or metal material in which openings are formed with a larger or smaller section.
According to one embodiment, the reinforcing structure assumes the form of a sheet with round, oblong perforations.
According to the prior art, the porous layer and the reinforcing structure are made independently of one another and are simply connected by gluing so as to be pressed against one another.
Acoustically, the performance of the acoustic treatment panels is improving. They are very strong with respect to certain stresses, for example such as compression forces in the transverse direction (direction perpendicular to the layers) and tensile stresses in a longitudinal direction (direction contained in the plane of the layers). However, these panels are not resilient and may break in the event of impact. Lastly, cracks may spread from a localized damage area along an uncontrolled path.
Consequently, under normal usage conditions, the acoustic treatment panels are satisfactory. However, in case of incidents, if the structure of the aircraft to which the acoustic panel is connected tends to deform, the stresses borne by the panel may cause fissures or cracks to appear, which may spread if the burdens are severe enough, thereby causing the panel to break into several parts. Although acoustic treatment becomes secondary in that situation, it is nevertheless important for the structure of the aircraft to be damaged as little as possible so as to allow it to reach its final destination.
This issue may be generalized to all walls of the aircraft that are made from a composite material, for example such as those forming the fuselage. As for the acoustic treatment panel, it is important to limit the spread of a crack in a given direction so that the structure of the aircraft is damaged as little as possible and to allow the aircraft to continue its mission.
According to a more specific issue, the parts made from a composite material of an aircraft having sections in a transverse plane with a closed perimeter, for example such as a section of the fuselage, an acoustic treatment panel for an air intake, or a section of the wing, have a structure with orbital reinforcements that limit the spread of cracks in a direction perpendicular to the transverse plane. However, these parts made from a composite material have a low resistance to limit the risks of a crack spreading in a transverse plane that may then extend over the entire circumference.
Document U.S. 2005/0112348 proposes a solution for reinforcing a panel that consists of affixing protruding strips on the surface of a panel. These strips may comprise non-metallic reinforcing fibers. This panel may not be used for applications where it is in contact with aerodynamic flows, due to the protruding elements. Furthermore, the nonmetallic fibers do not limit the spread of cracks, as they “break” just as much as the fibers of the layers making up the panel.